This invention relates to a method for cross-linking polyethylene with an ionizing radiation, and more particularly, relates to a method for cross-linking polyethylene, characterized in that polyethylene is blended with dipropargyl maleate and a copolymer selected from the group consisting of an ethylene-acrylonitrile copolymer, an ethylene-acrylic ester copolymer, an ethylene-substituted acrylic ester copolymer, an ethylene-vinyl acetate copolymer, an ethylene-substituted vinyl acetate copolymer and mixtures thereof, and the resulting mixture being irradiated with an ionizing radiation.
It has been known that the mechanical, chemical and thermal properties of polyethylene can be improved through cross-linking. Especially, an irradiation method by ionizing radiation has an advantage in that cross-linking treatment can be carried out at low temperatures in the region of room temperature. However, when this cross-linking treatment of polyethylene is effected without using a special cross-linking agent, a large amount of ionizing radiation is required, and therefore side reactions such as cutting of main chains in polyethylene can not be ignored. For example, the cutting of main chains results in not only a decrease in cross-linking efficiencies but also a lowering of mechanical strength of cross-linked material. The irradiation of polyethylene with a great amount of accelerated electron rays also leads to a considerable decrease in the quality of cross-linked polyethylene because the accumulation of electric charge gives rise to the Lichtenberg breakdown of polyethylene and the elevation of temperatures induces the blowing of the polymer. Moreover, the irradiation of polyethylene with a great amount of radiation increases costs for maintaining and operating apparatus such as shielding apparatus. Therefore, development of a technique for reducing total dose of radiation is increasingly required for the spread of a cross-linking method by radiation.
In the prior art, it has been known that an amount of ionizing radiation for cross-linking a polymer can be reduced by adding a polyfunctional monomer, such as divinyl benzene or diethylene glycol diacrylate having plural double bonds per molecule to the polymer. However, according to this method, the promotion of cross-linking has not been satisfactory, and consequently a highly cross-linked polymer could not be obtained without a large amount of total dose of ionizing radiation. Therefore, the problem of reducing total dose of ionizing radiation remains unsolved in the case of cross-linking a polymer. There also remain several problems for radiation installation and safety in commercially practicing the method comprising contacting a polymer to be cross-linked with a gas or a low-boiling liquid.
We have found that a variety of polymers including an ethylenic polymer can be efficiently cross-linked by blending them with a high-boiling acetylenic compound having at least one ethynyl (CH.tbd.C--) per molecule, followed by irradiating the blend with ionizing radiation (Japanese Patent Publication No. 44577/1974). However, the known method requires a considerably greater amount of an ionizing radiation to obtain a highly cross-linked polymer, because the formation rate of the cross-linked polymer is radpily lowered as the total dose of radiation and greater cross-linking percentage increase.